The electronic parts used in the CPUs (Central Processing Units), etc. of servers and personal computers, etc. have the structure that a heat spreader of a material of high thermal conductivity, such as copper or others, is arranged with a thermal conductive sheet of an indium sheet or others provided immediately on the semiconductor element so as to efficiently radiate the heat generated by the semiconductor element.
However, the recent large demand increase of rare metal has raised the price of indium, and substitute materials which are less expensive than indium are expected. In terms of physical properties, the thermal conductivity of indium (50 W/m·K) cannot be said to be high. For more efficient radiation of the heat generated by the semiconductor element, materials of higher thermal conductivities are expected.
In such background, as a material having a thermal conductivity higher than indium, a linear structure of carbon atoms represented by carbon nanotube is noted. Carbon nanotube has not only a very high thermal conductivity (1500 W/m·K) but also good flexibility and electric conductivity. Carbon nanotube has high potential as a radiation material.
As a thermal conduction sheet, a thermal conductive sheet having carbon nanotubes distributed in a resin is disclosed in, e.g., Japanese Laid-open Patent Publication No. 2005-150362. A thermal conductive sheet having carbon nanotube bundles grown oriented on a substrate and buried in a resin is disclosed in, e.g., Japanese Laid-open Patent Publication No. 2006-147801.
Carbon nanotube is noted as an interconnection material to be used in semiconductor devices, etc. The copper interconnection presently dominantly used in integrated circuit devices has many explicit problems of reliability degradation, etc. due to electromigration as the devices are increasingly downsized. Then, carbon nanotube, which has good properties, such as good electric conductivity, permissible current density, which is about 1000 times higher than that of copper, ballistic electron transportation property, etc., is expected as a next generation interconnection material.
As an interconnection using carbon nanotube, proposals of vertical nanotube interconnections using vias are made (refer to, e.g., M. Nihel et al., “Electric properties of carbon nanotube bundles for future via interconnects,” Japanese Journal of Applied Physics, Vol. 44, No. 4A, 2005, pp. 1626-1628.
The following are examples of related: Japanese Laid-open Patent Publication No. 2006-303240; Japanese Laid-open Patent Publication No. 09-031757; Japanese Laid-open Patent Publication No. 2004-262666; Japanese Laid-open Patent Publication No. 2005-285821; Japanese Laid-open Patent Publication No. 2006-297549; Japanese Laid-open Patent Publication No. 2006-339552; and Japanese Laid-open Patent Publication No. 2003-238123.
The thermal conductive sheet disclosed in Japanese Laid-open Patent Publication No. 2005-150362 has carbon nanotubes simply distributed in a resin, and thermal resistances are generated at the joints between the distributed carbon nanotubes. Carbon nanotube has the characteristic that the thermal conductivity along the direction of orientation of carbon nanotube is minimum, but the thermal conductive sheet disclosed in Japanese Laid-open Patent Publication No. 2005-150362, in which the orientation direction of the carbon nanotubes is disuniform, has failed to make the best use of the high thermal conductivity of the carbon nanotube. In this point, the thermal conductive sheet disclosed in Japanese Laid-open Patent Publication No. 2006-147801 has carbon nanotube bundles grown oriented on a substrate, and can realize higher thermal conductivity than the thermal conduction sheet disclosed in Japanese Laid-open Patent Publication No. 2005-150362.
However, the inventors of the present application examined the thermal conductive sheet disclosed in Japanese Laid-open Patent Publication No. 2006-147801 and have found that aggregations and biases take place between carbon nanotube bundles when a resin is filled between the carbon nanotubes with a result that the orientation and the uniformity of the carbon nanotubes are impaired, and the thermal conductivity cannot be realized as expected. In this structure, the radiation in the vertical direction (the direction perpendicular to the surface of the sheet) can be ensured to some extent, but it is difficult to ensure the radiation in the horizontal direction (the direction parallel to the surface of the sheet). That is, the thermal conductivity of the resin as a whole is about 1 (W/m·K) and is lower by about 3 places in comparison with the vertical thermal conductivity of carbon nanotube. The radiation effect in the horizontal direction is very low.
Preferably, the interconnection material can form not only interconnection structures connected in the vertical direction as disclosed in M. Nihel et al. but also interconnection structures connected in the horizontal direction. However, the horizontal interconnection using carbon nanotube has not been realized yet because there are many difficulties in controlling the horizontal growth of the nanotube, and additionally, it is difficult in terms of the process to form electrode blocks interconnecting the via interconnections and the horizontal interconnections, which are the starts of the horizontal interconnections.